A "brushless" motor includes a permanent magnet rotor and a stator made up of a number of windings that may be connected in a star configuration, or completely independent from each other. In the first case there exist a number of external terminals equal to the number of motor's phases (eventually +1 if the star center must be accessed). In the second case reference is made to motors with independent phases and both terminals of each winding are available externally. These motors are commonly used in hard and floppy disk drives, DVD (digital video disk) drives, in tape video recorders, CD players, etc.
In the majority of cases, brushless motors are three-phase and the driving circuit generally comprises integrated circuits whose output stages drive the phase windings. The integrated circuits may either comprise a three-phase full-wave half-bridge circuit in the case of motors connected in a star configuration, or three full-bridge circuits in the case of independent phase motors.
FIG. 1a shows a three-phase independent phase brushless motor and the relative independent full-bridge output stages that drive the respective windings. FIG. 1b illustrates the output stage required for driving a three-phase brushless motor with windings connected in a star configuration.
By way of simplification, let us consider the typical case of a three-phase motor driven in a bipolar mode in which, at each instant, two phase windings are powered, while the third phase winding is momentarily unpowered (full-bridge or half-bridge output node in a state of high impedance). The phase windings are switchingly driven according to a cyclical sequence which must be synchronized with the rotor's instantaneous position. In a bipolar driving mode, the position may be determined by analyzing the back-electromotive-force (BEMF) monitored on the winding that is momentarily unpowered, or sensed by Hall-effect sensors (a more expensive approach that is seldom used). Normally, such a BEMF monitoring detects the zero crossing of the BEMF that has a sinusoidal or in any case periodic waveform, generally referred to as "zero-cross" instant. The time interval between two successive zero-crosses is indicated with Tc.
Alternative driving modes for this type of motor are, respectively, the so-called unipolar mode and the tripolar mode. In the unipolar mode the sensing of the rotor's position may be made in the same way as in the bipolar mode. In the tripclar mode the sensing of the rotor's position cannot be made according to the above noted techniques because all of the motor's windings are always powered. In fact, in a tripolar mode three voltages (or currents), generally sinusoidal, out-of-phase by 120 electrical degrees among each other, and generating a stator rotating field, are forced on the three windings, respectively, of a three-phase motor.
In a tripolar mode, the sensing of the rotor's position generally requires the use of sensors or of electronic circuits that provide a de-facto "reconstruction" of the induced BEMF voltage on the windings, once the motor's electrical parameters are known. An approach of this type is disclosed in the European patent application No. 96830440.2 assigned to the assignee of the present invention.
Information on the rotor's instantaneous position is necessary for driving a synchronous motor, such as a brushless motor. It is fundamental for optimizing the driving because the system's efficiency is a function of the phase relationship between the driving voltage (or current) and the rotor's position.
The methods for monitoring the instantaneous rotor's position used in a bipolar or in an unipolar driving mode are unsuitable in the case of a tripolar mode. This is so because each winding is constantly subjected to a driving voltage, thus impeding the monitoring of the BEMF induced on the winding. According to the prior art, this inability was addressed by employing position sensors, for example, Hall effect sensors or, as an alternative, relatively complex electronic circuits capable of reconstructing a BEMF signal once the motor's electrical parameters (resistance and inductance) are known.